This invention relates to a process for the production of a magnetic recording material consisting of a non-magnetic layer support and, cast thereon, a magnetic dispersion consisting essentially of a mixture of at least two different types of finely divided magnetic pigments finely dispersed in a polymeric binder.
It is known that the properties of magnetic recording materials have to meet stringent requirements both in regard to mechanical stability and in regard to electroacoustic properties In addition to many other parameters, high surface homogeneity of the magnetic layer is an essential requirement for adequate performance. To obtain these properties, it is necessary inter alia to choose suitable polymeric binders, dispersants and magnetic pigments.
To produce the recording materials, the finely divided magnetic pigments are normally processed with the usual additives, such as dispersion aids and, optionally, non-magnetic fillers, and the binders dissolved in organic solvents in a dispersion unit, for example a ball mill of the pot or stirred type, to form a dispersion. The dispersions are generally prepared by processes employed for the production of lacquers. In there processes it is very important not only to observe the chemical composition very accurately but also to employ a complicated mechanical process in dispersion apparatus which will be described below. This and the physicalchemical properties of the components of the dispersion and their interaction determine the internal structure of the dispersion, of which the degree of dispersion and the degree of agglomeration of the individual particles, in particular of the magnetic pigments, play a decisive role. Electrostatic and magnetostatic forces between the pigments may lead to the formation of agglomerates which only come to light later in the manufactured magnetic recording carrier. The dispersion thus prepared is then applied to the layer support by means of standard coating machines.
The major characteristics which determine the quality of the dispersion are the degree of dispersion or degree of agglomeration, the particle density and the viscosity. Another important factor is the stability of the dispersion, which depends to a large extent on the time in that it may change during the time that elapses between the preparation of the lacquer and casting of the dispersion. The dispersion is normally prepared in three stages:
1. Preliminary grinding of the magnetic pigment is carried out with or without solvent in the presence of a wetting agent and optionally a dissolved polymeric binder.
2. In the predispersion stage, the pigment deposit described under 1 is dispersed into a concentrated binder solution, whereupon other of the additives mentioned above may be added.
3. The required degree of dispersion of the magnetic dispersion is finally obtained in a more prolonged process of fine dispersion.
At the present time, the quality of the dispersion is tested during production by preparing a sample of cast dispersion which is substantially similar to a finished magnetic recording carrier, and its quality is then tested. Subsequent treatment of the dispersion depends on this test. It will be obvious that such testing is time-consuming, expensive and inaccurate since it is not possible to have any information about the state of the dispersion immediately before and during application of the dispersion to a support.
EP 0146 015 describes a rapid method for determining the degree of dispersion in flowing two-phase systems. The pressure drop in the dispersion as it flows along a specified length of path is measured. This method would appear to be too inaccurate for the purpose stated above.
A process for measuring the electrokinetic Zeta potential of a dispersion is disclosed in DE-PS 23 37 165. A sample stream is continuously removed from the dispersion under investigation and transferred to a separating cell in which the stream of sample is exposed to a magnetic field and divided up into several partial streams. These partial streams are then passed through separate measuring cells in which the solids content of the individual partial stream is determined. This method of measuring determines the electrostatic charge of the pigment in relation to the polymeric binder system and characterises the stability of the dispersion but not the degree of dispersion.
A process for determining the degree of dispersion of magnetic dispersions is known from Journal Dispersion Science and Technology, 7 (2), pages 159 to 185 (1986). This method uses the uptake of mercury on the surface of the magnetic pigments to assess their volume. This method is too elaborate and unsuitable as a rapid test.
According to EP 0 103 655, the dispersion flowing through a tube is exposed to a magnetic alternating field of variable frequency and the results are used to determine the susceptibility. This method of determination depends to a great extent on the coercivity of the magnetic pigments and on the rate of throughflow and is therefore unsuitable as a universal measuring method for use in production processes. A similar apparatus is described in Japanese Application No. 58-76758.
An apparatus for measuring the aggregation of particles against a wall or with one another is described in DE-OS 29 29 018. The particles are in this case dispersed in a stream of liquid or gas. The liquid or gaseous multiphase system is directed towards a transparent wall and illuminated. The light which is scattered, reflected or attenuated by absorption is directed towards a detector and assessed. Measurement of the aggregation of particles with one another or against the wall is only possible with highly diluted dispersions which are transparent. It is not suitable for highly concentrated dispersions containing magnetic particles, since such dispersions are opaque. With certain magnetic recording materials , two or more layers containing different pigments are cast over on another to obtain the desired electroacoustic properties. Other magnetic recording materials comprise only one magnetizable layer, although mixtures of different magnetic pigments, for example chromium dioxide and iron oxide or ferrite and/or metal powders or metal powder alloys, are used in this magnetizable layer. Recording supports of this type are known, for example, from the publications DE 19 29 376 (U.S. Pat. No. 3,653,962), 19 55 699 (U.S. Pat. No. 3,824,128), 25 38 005 (GB 1 484 958) and 33 22 746 and from OS 47 16 076 U.S. Pat. No. 4,716,076. The processes normally used for fine dispersion of the magnetic pigments are known from numerous publications, for example from the book entitled "Technologie der Magnetbandherstellung (Technology of Magnetic Tape Manufacture)" (Akademie - Verlag Berlin, 1981) and from Applicants' pending U.S patent application Ser. No. 07/242,521, filed Sept. 12, 1988. However, the magnetic recording materials thus produced often do not have optimal and/or reproducible electroacoustic properties. The various polymeric binders and dispersion aids for improving the properties are all described in numerous patent applications.
Other publications put forward technological proposals to improve dispersion, including for example step-by-step addition of the binders, dispersants or solvents Recently, many magnetic recording materials have also been required to show a high packing density. This requirement can largely be satisfied by reducing the binder content, albeit at the expense of dispersion stability.